1. Field of the Invention
This invention relates to all-gear differentials of the type commonly referred to as “limited-slip” designed primarily for automotive use and, more particularly, to such differentials designed for use in vehicles where efficiency, space, cost, and weight are critical.
2. Description of Related Art
While there are many types of limited-slip differentials, some of the most commercially successful have been the all-gear differentials based upon the designs of Vernon E. Gleasman, and the most efficient of these have been those based upon his crossed-axis design that has been identified commercially as the Torsen®-Type 1 differential. A recent improvement of such known limited-slip differentials using crossed-axis planetary gearing was disclosed in U.S. Pat. No. 6,783,476 (“Compact Full-Traction Differential”, assigned to the same assignee as the present invention and identified by the trademark “IsoTorque”), incorporated by reference herein. The improved differential disclosed in the just-identified patent is smaller in both size and weight than earlier designs of other prior art crossed-axis differentials, and it is less costly to manufacture, while meeting similar load-carrying specifications.
All traditional Gleasman crossed-axis differentials include pairs of unusual balance (combination) gears, e.g., 131, 132 and 131a, 132a in FIGS. 1A and 1B, that (a) mesh with each other through spur-gear portions 133 formed at each end and (b) mesh with the side-gear worms 141, 142 through helical teeth formed in hourglass shaped worm-wheel portions 134 positioned between the two spur teeth ends. A key feature of Gleasman all gear cross-axis differentials, including the older designs (e.g., Torsen®-Type 1), is the relationship of each side gear, intended to act as a “worm”, with “worm-wheel” teeth on the central portions of each of the differential's balance gears.
A “worm” is traditionally a cylindrical gear with teeth in the form of a screw thread that mates with a larger gear generally identified as a “wormgear” or as a “worm-wheel”, and that latter term is used herein. However, as used in Gleasman-type differentials, the side-gear worm is the larger of the two gears. In traditional worm/worm-wheel gearing, there is a mechanical advantage as energy is transferred from the worm to the worm-wheel and a concomitant mechanical disadvantage when energy is transferred from the worm-wheel to the worm. This same mechanical advantage/disadvantage relationship is also true in regard to the transfer of energy between the side-gear worm and the balance gear worm-wheel of the Gleasman-type differentials as just discussed above and as disclosed herein.
In conventional differentials, when one drive wheel of a vehicle loses traction, most of the engine torque is immediately delivered to the slipping wheel. With Gleasman-type differentials, the mechanical disadvantage created by the worm-wheel/worm connection from the engine to the wheel constrains the excess slipping of the low-traction wheel. This same connection, when operating in the worm/worm-wheel direction, enhances the response of the differential to the changes in drive wheel speeds when the vehicle is turning corners and the outside wheels are traveling over a longer distance than the inside wheels within the same time period.
The geometric requirements for a smooth rolling gear-mesh normally restrict the tooth ratios (i.e., ratio of the number of teeth in one member of a gear pair to the number of teeth in the other member) of true worm/worm-wheel gear sets to a ratio of at least 3.5:1, and much higher tooth ratios are normally designed for this class of gear set. This ratio limitation is true for straight flank worms of the screw thread type as well as for involute helicoid worms of the generated type. Numerous geometric interferences will normally result from any attempt to design worm/worm-wheel gear sets with a ratio lower than 3.5:1.
However, in view of the relatively small package size and relatively high strength requirements of the gear members in a crossed-axis differential, the optimal worm/worm-wheel ratio would ideally fall into the gear ratio range of 1.5:1 and 2.5:1, but none of the prior art gears are able to meet these ratios. Therefore, in actual practice, the side gear “worm” teeth and the balance gear “worm-wheel” teeth of prior art crossed-axis differentials have not been executed as actual worm/worm wheel designs, but rather as crossed helical gear sets, with both elements having simple helical gear geometry. The serious limitation of this approach is that crossed helical gear sets have instantaneous “point” contact, rather than broad area contact patterns, and thus are susceptible to loading limitations and accelerated wear.
The crossed helical gear geometries in the existing art are also quite limited in their frictional component, as they operate primarily in rolling contact over their very limited contact area. Because the effectiveness of torque transfer to the wheel with greater traction depends upon this frictional component, it would be desirable to increase friction in this critical side gear/balance gear mesh. Such frictional increases would have little effect upon overall driveline efficiency, since the low relative rotational speed of this set represents only differentiation in wheel speed, normally falling in the 0-20 rpm range in practice.
A partial change from the traditional helical shape is disclosed in above-identified U.S. Pat. No. 6,783,476. That patent discloses helical worm/worm-wheel teeth having a “supra-enveloping” contact pattern. Namely, the worm-wheel portions of the balance gears still have the traditional helical-gear shape (an involute helicoid form being cut with a conventional straight-sided hob), while the meshing worms (i.e., the side gears) have mating “inverse-involute” teeth that are cut with an involute hob. That patent, as well as other prior art, also suggests the use of “closed-end” side gears.
Conventional crossed-axis helical gears are cut by a hob with straight-sided teeth, the hob being rotated with a combination of plunge and axial feed. Conventional worm-wheels are also cut by a hob with straight-sided teeth, but the hob is rotated with only a plunge-feed and no axial feed.
As indicated above, all-gear crossed-axis differentials include unusual balance gears that (a) mesh with each other through spur teeth formed at each end and (b) mesh with the side gears through helical teeth formed between the two spur teeth ends. During assembly, these unusual teeth must be positioned in proper mesh and orientation to assure equal load sharing. This orientation process is referred to as “timing”.
In all prior art designs, the gears have a mixture of odd and even numbers of teeth. A typical prior art example is as follows: the number of spur teeth at each end of the balancing gears is 18, the number of teeth in the worm-wheel of each balancing gear is 7, and the number of teeth in each side gear is 13. These unusual prior art tooth numbers are not created haphazardly but rather are particularly chosen in part to combat special gear set wear problems associated with the point or line contact characteristics of crossed helical gear sets. Nonetheless, these differing gear numbers create complicated timing problems. For instance, all prior art designs require that timing marks be placed on each combination gear and that careful attention be made during assembly to an instruction chart. The order of gear assembly is indicated as well as the individually different distances that the mark must be rotated for each gear as it is assembled, etc. The prior art instructions, for example, “. . . [T]he internal loads will not be evenly balanced among the gears, and some will be severely overloaded. This will lead to eventual failure, often catastrophic”, warn that incorrect timing can have dangerous results.